A composite right/left-handed transmission line (hereinafter, referred to as a CRLH (Composite Right/left-Handed) transmission line) has been known as one of metamaterials. The CRLH transmission line is configured by substantially periodically inserting capacitive elements in series branch of the transmission line and substantially periodically inserting inductive elements in shunt branch, at intervals sufficiently smaller than the wavelength so as to have a negative effective permeability and a negative effective dielectric constant in a predetermined frequency band. In recent years, a nonreciprocal phase shift CRLH transmission line obtained by adding a nonreciprocal transmission function to the CRLH transmission line has been proposed (See, for example, Patent Documents 1 to 3). The nonreciprocal phase shift CRLH transmission line is able to exhibit a positive refractive index when electromagnetic waves having an identical frequency propagate in the forward direction and to exhibit a negative refractive index when the electromagnetic waves propagate in the backward direction.
When the transmission line resonator is configured by using the nonreciprocal phase shift CRLH transmission line, the resonator size can be freely changed without changing the resonance frequency. Further, the electromagnetic field distribution on the resonator is similar to the electromagnetic field distribution of a traveling wave resonator. Therefore, by using a transmission line resonator having the nonreciprocal phase shift CRLH transmission line, a pseudo traveling wave resonator can be configured such that an amplitude of the electromagnetic field of the pseudo traveling wave resonator is uniform and a phase of the electromagnetic field of the pseudo traveling wave resonator linearly changes with a constant gradient along the transmission line. In this case, the phase gradient of the electromagnetic field distribution on the resonator is determined by the nonreciprocal phase shift characteristic of the transmission line configuring the resonator. Hereinafter, the transmission line apparatus using the nonreciprocal phase shift CRLH transmission line is referred to as a nonreciprocal transmission line apparatus.
The metamaterials have been a very interesting important theme in the field of applications to antennas for more than a decade. The nonreciprocal CRLH metamaterial has been proposed for the purpose of applications to directional leaky wave antenna using the CRLH transmission line until now. Moreover, recently, an antenna based on the pseudo traveling wave resonator highly developed from the zeroth-order resonator (See, for example, Non-Patent Document 1) has been proposed, so that the gain and the directivity are increased in spite of compactness as compared with the conventional leaky wave antenna.
Many ones of the nonreciprocal transmission line apparatuses that have been proposed until now adopt such a structure that a ferrite rod perpendicularly magnetized is embedded under the strip line at the center of the composite right/left-handed transmission line apparatus configured of the conventional microstrip line. In this case, the direction of the radiation beam from the antenna apparatus having the pseudo traveling wave resonator configured of the nonreciprocal transmission line apparatus is determined by the phase gradient of the electromagnetic field distribution on the resonator. Moreover, if the ferrite is a soft magnetic material, the nonreciprocal phase shift characteristic of the transmission line is changed by changing the magnitude or the direction of an externally applied magnetic field, and beam scanning can consequently be performed.
For example, Non-Patent Document 1 proposes application of the pseudo traveling wave resonator having the nonreciprocal transmission line apparatus to a beam-scanning antenna. The beam scanning antenna having the pseudo traveling wave resonator has such a drawback that the operation band is narrow, however, the beam scanning antenna has higher radiation efficiency than that of the conventional leaky wave antenna. Further, the problem of the occurrence of beam squint, which is such a phenomenon that the radiation beam direction changes in accordance with the frequency change of the propagation signal, is largely reduced.
The beam squint is a phenomenon well known in the conventional phased array antenna, or such a phenomenon that the beam radiation angle fluctuates depending on the frequency. The operation bandwidth is disadvantageously suppressed by this (See, for example, Non-Patent Document 6). In the ordinary array antenna, the main cause of the beam squint is in the dispersibility of the delay element. As one method for solving this, there can be enumerated a tunable time delay element used as an active CRLH delay element disclosed in Non-Patent Document 8. In the case of the CRLH metamaterial, this kind of compensation circuit is meaningless, and it has been possible to reduce the beam squint only in the upper bands of the series resonance frequency of the series branch and the parallel resonance frequency of the shunt branch (See, for example, Non-Patent Document 7).